Attach source range information to parsed YAML nodes.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index 9a4d3e34930a1b091f4cb0a2026d4b01cc385fe0..8c29253492e1c754900e84665afdd90622c3ac0a 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -357,6 +357,9 @@ class Pubspec {
         sourceName = _sources.defaultSource.name;
         versionConstraint = _parseVersionConstraint(spec, "$field.$name");
       } else if (spec is Map) {
+        // Don't write to the immutable YAML map.
+        spec = new Map.from(spec);
+
         if (spec.containsKey('version')) {
           versionConstraint = _parseVersionConstraint(spec.remove('version'),
               "$field.$name.version");